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Full text of "Handbook Of Chemical Engineering - I"

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40
CHEMICAL ENGINEERING
the furnace, one chute supplying 30 sq. ft. 'of grate. Heating surfaces must be frequently cleaned.
Bagasse may now be burned without auxiliary fuel, on a hearth leading to brick-walled passages. About 0.3 in. draft will give a rate of combustion of 250 to 300 lb. Tan bark also requires a high rate of combustion. 1 Ib. of ground bark yields slightly over 2 lb. of spent tan. Black ash liquor is available in soda-process pulp mills. It is burned in rotary extension furnaces with auxiliary fuel: high concentration of liquor decreases the consumption of the latter. The ash is reclaimed and the steam produced is a by-product.
Gas Fuels.—The important gas fuels are listed below, with typical analyses as given by Lucke:
	Analysis by volume per cent								Density,	B t u
Name									pounds per cubic foot, 32°F.	per pound (low value)
	CH4	H2	CeHe	CO	02	C2H4	CO2	N2		
Natural gas ............	98.4		0.1	0.25	0.25				0. 0446	21,301
Coal gas, coke oven gas	34.3	42.00	2.0	6.00	1.10	2.0	2.50	10.10	0. 0403	14,384
Blast-furnace gas .......	0.2	2.74		28.60			11.39	57.06	0.0812	1,323
Water gas ..............	4.41	45.57		44.85	0.50	0.1	4.45	0.12	0.0456	7,228
Producer gas ...........	0.2	15.00		26. 10	0.20		5.30	53.2	0.0696	1,934
Coal gas is the product of slow distillation of coal with exclusion of air. Blast-furnace gas results from the combustion of coal in a restricted supply of air: its ideal composition would be, by weight, CO, 34.4; N, 65.6. Water gas is made by the action of steam on incandescent coal, and should consist ideally of H and CO only: the process is necessarily intermittent. Producer gas is a combination of blast-furnace gas and water gas, the supply of steam being so limited as to permit of a continuous process. In all of the manufactured gases, the hydrocarbons (CH4, C6H6, C2H4) come from the coal either directly or as the result of a breakdown of other hydrocarbons. The hydrogen in coal gas and blast-furnace gas has a similar origin. The CO and CO2 in coal gas are due to a partial (and objectionable) combustion of the carbon in the coal.
For steam boilers, fuels are natural gas and blast furnace gas. A large number of small burners (about 30 hp. each) is used for the former. Burners are thoroughly distributed. As with fuel oil, they may be directed either slightly downward toward the rear or may face from the bridge wall toward the front. The combustion space should be large. The air supply is drawn in around the gas jet. High efficiency is easily possible. For blast furnace gas, use a furnace space of 2 cu. ft. per rated horsepower, 0.8 sq. in. of gas passage area per horsepower and 2 in. blast (based on 6 to 8 in. pressure in main). Provision must be made for removal of dust. With all gas burners, the gas and air supply should be separately controllable.
Powdered Coal.—The fuel is blown into the furnace so as to burn in suspension. Furnace construction is much the same as for oil, and combustion is very perfect. Dryers are employed to drive off moisture before grinding. On account of its lower moisture content, run-of-mine coal may be preferred to slack, in spite of its higher price. The chief cost is for fixed charges in the grinding and feeding equipment, and for power. From 50 cts. to $1 per ton seems to be